The present invention relates to optical ring networks, and more particularly, to fault recovery systems in optical wavelength division multiplexing ring networks.
A typical ring network includes nodes, each node having a unique address. A demand will request that specific information be transmitted from a sending node to a receiving node. Traffic between different sending node/receiving node pairs is assigned to different wavelength channels, each of which may be considered either even or odd.
Adjacent nodes are interconnected by at least two fiber links, one being a clockwise-directed fiber for transmittal of even channels, and a second being a counterclockwise-directed fiber for transmittal of odd channels. The clockwise-directed fibers and the nodes between them constitute a clockwise-directed ring, and the counterclockwise-directed fibers and the nodes between them constitute a counterclockwise-directed ring.
For wavelength division multiplexing ring networks, each node has the apparatus for being a sending node and for being a receiving node. For this purpose, each node typically includes a frequency multiplexer and demultiplexer. The multiplexer is responsive to the demand so that if that node is the sending node specified in the demand, the information will be encoded, wavelength multiplexed to the appropriate channel, and added to the data stream arriving at that node. The demultiplexer is responsive to the data stream arriving at the node, so that it decodes each signal, determines if that node is the destination node and if so, drops the signal.
Occasionally, a link fails for one or more wavelength channels. In conventional systems for network recovery, the link failure is broadcast to each node. A switch on each node then reassigns signals from inoperative to operative wavelength channels. Such a system is expensive and cumbersome because of the broadcasting of link failure to each node and because each node must include a switch for each channel.
A network according to the principles of the invention includes a link failure detector and a failure correction apparatus. The link failure detector is in at least one of the links for detecting whether transmission of a signal intended for transmission between the pair of adjacent nodes which the link is between was successful or unsuccessful. The link failure detector also produces a link transmission signal indicative of success or lack of success. The link transmission signal is intended for transmission to the pair of nodes adjacent to the link.
The failure correction apparatus is in each node adjacent to a link having the link failure detector. The failure correction apparatus is responsive to the link transmission signal produced by the failure correction apparatus, so that in response to a link transmission signal indicative of unsuccessful transmission, any signal in either network arriving subsequently at the node is diverted to the other network, and so that in response to a link transmission signal indicative of successful transmission, any signal in either network arriving subsequently at the node is permitted to continue in the same network in which it arrived. Any signal subsequently produced at the node for transmission into either network is treated by the fault correction apparatus as a signal in the same network arriving subsequently at the node.
Such a system successfully diverts signals around any break in the network, and avoids the broadcasting of link failure to each node since only adjacent nodes are affected. Thus, it is simpler and less costly. Furthermore, each node need not include a switch for each channel. A fault correction apparatus on the node switches all nodes from one network to the other, regardless of which channels failed.